Methods and systems for neutralizing hydrogen sulfide during drilling

ABSTRACT

A method to neutralize hydrogen sulfide in a wellbore during drilling operations is provided. The method comprising the steps of mixing an amount of scavenger into a drilling mud to produce a scavenger-containing drilling mud such that the scavenger-containing drilling mud has a scavenging capacity, wherein the drilling mud is at a target pH, and introducing the scavenger-containing drilling mud into the wellbore during drilling operations, where the scavenger-containing drilling mud is operable to work with a drill bit to drill the wellbore, where the amount of scavenger in the scavenger-containing drilling mud is operable to irreversibly react with hydrogen sulfide present in the drilling mud to produce a scavenged hydrogen sulfide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. Non-Provisional patentapplication Ser. No. 15/260,387 filed on Sep. 9, 2016. For purposes ofUnited States patent practice, the non-provisional application isincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to methods and systems for the removal ofhydrogen sulfide. More specifically, the invention relates to methodsand systems for the removal of hydrogen sulfide and that monitorhydrogen sulfide in the fluids.

Description of the Related Art

During drilling in a formation that has a high amount of hydrogensulfide (H₂S), such a high H₂S environment, H₂S can be released at thesurface causing problems to the surrounding environment and interruptingthe drilling process. In addition, the release of H₂S can negativelyaffect safety and performance. Current methods for removing H₂S fail tomonitor or control the concentration of H₂S removal compounds opening upthe risk that too little H₂S removal compounds are added and thusallowing H₂S to be released. In addition, many current hydrogen sulfidescavengers can harm the environment due the presence of harmfulcomponents, such as zinc. Other hydrogen sulfide scavengers canprecipitate a residual that can cause formation damage, such as thosethat contain iron compounds.

SUMMARY OF THE INVENTION

This invention relates to methods and systems for the removal ofhydrogen sulfide. More specifically, the invention relates to methodsand systems for the removal of hydrogen sulfide and that monitorhydrogen sulfide in the fluids.

In a first aspect of the invention, a method to neutralize hydrogensulfide in a wellbore during drilling operations is provided. The methodincludes the steps of mixing an amount of scavenger into a drilling mudto produce a scavenger-containing drilling mud such that thescavenger-containing drilling mud has a scavenging capacity, where thedrilling mud is at a target pH, and introducing the scavenger-containingdrilling mud into the wellbore during drilling operations, where thescavenger-containing drilling mud is operable to work with a drill bitto drill the wellbore, where the amount of scavenger in thescavenger-containing drilling mud is operable to irreversibly react withhydrogen sulfide present in the drilling mud to produce a scavengedhydrogen sulfide.

In certain aspects, the method further includes the step of adding pHbuffer to the drilling mud before the step of mixing an amount ofscavenger into a drilling mud such that the pH target of thescavenger-containing drilling mud is in a range between 10.5 and 11.0.In certain aspects, the amount of scavenger is in excess of 4 lb/bbl. Incertain aspects, the scavenging capacity between 800 mg/l and 1200 mg/l.In certain aspects, the method further includes the steps of monitoringthe pH of the scavenger-containing mud, and adding additional pH bufferto the scavenger-containing mud to maintain a target pH between 10.5 and11.0. In certain aspects, the pH is monitored every 30 minutes. Incertain aspects, the pH is monitored with a Pen Type pH monitor. Incertain aspects, the scavenger is triazine. In certain aspects, themethod further includes monitoring a scavenging capacity of thescavenger-containing mud; and adding an additional amount of scavengerif the scavenging capacity is below a depleted scavenger level.

In a second aspect of the invention, a method to neutralize hydrogensulfide in a contaminated mud at a surface during drilling operations isprovided. The method includes the steps of introducing the contaminatedmud to a treatment loop, where the contaminated mud exits a wellboreprior to being introduced to the treatment loop. The treatment loopincludes a pH buffer injection point, the pH buffer injection pointconfigured to inject a pH buffer into the contaminated mud to produce ahigh pH mud at a target pH, a sample point, the sample point configuredto measure the pH of the high pH mud, and a scavenger injection point,the scavenger injection point configured to inject an amount ofscavenger. The method further includes the step of injecting pH bufferinto the contaminated mud at the buffer injection point to produce thehigh pH mud, measuring the pH from the sample point, injecting theamount of scavenger into the high pH mud to produce a treated mud,introducing the treated mud into an agitator tank, the agitator tankconfigured to circulate the treated mud for a treatment time to producea clean mud, and introducing the clean mud to a rig tank, the rig tankconfigured to supply the clean mud to the wellbore.

In certain aspects, the amount of scavenger is in excess of 4 lb/bbl. Incertain aspects, the scavenging capacity between 800 mg/l and 1200 mg/l.In certain aspects, the agitator tank includes an agitator, the agitatorconfigured to create turbulence in the agitator tank, and a circulationpump, configured to withdraw a portion of the treated mud and return thewithdrawn portion to the agitator tank. In certain aspects, the samplepoint includes a pH monitor. In certain aspects, the scavenger istriazine. In certain aspects, further includes the steps of monitoring ascavenging capacity in the treated mud, and adding an additional amountof scavenger at the scavenger injection point if the scavenging capacityis below a depleted scavenger level.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescriptions, claims, and accompanying drawings. It is to be noted,however, that the drawings illustrate only several embodiments of theinvention and are therefore not to be considered limiting of theinvention's scope as it can admit to other equally effectiveembodiments.

FIG. 1 is a schematic diagram of a system according to an embodiment ofthe invention.

FIG. 2 is schematic diagram of a system according to an embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

While the invention will be described with several embodiments, it isunderstood that one of ordinary skill in the relevant art willappreciate that many examples, variations and alterations to theapparatus and methods described herein are within the scope and spiritof the invention. Accordingly, the exemplary embodiments of theinvention described herein are set forth without any loss of generality,and without imposing limitations, on the claimed invention.

Advantageously, the present invention provides methods and systems forremoving hydrogen sulfide from a drilling mud without the need to sendthe hydrogen sulfide gas exiting the well at the surface to a flare. Themethods and systems of the present invention neutralize the hydrogensulfide from the well. Hydrogen sulfide can be released from theformation into the well. Hydrogen sulfide released from the well can mixwith the drilling mud that can be returned from the wellbore.

As used herein, “drilling mud” refers to any drilling fluid suitable foruse in drilling the wellbore. The drilling mud can be an aqueous-baseddrilling mud. Where other terms are used to describe the drilling mud bythe presence of sulfides, pH buffer, or scavenger, it should beunderstood that the “base” drilling mud remains the same, with the addedcomponents.

As used herein, “drilling operations” refers to the use of a drill anddrill bit to drill a wellbore into a formation. The use of drillingoperations encompasses all formation types, drills, rigs, and processeswhere hydrogen sulfide can be present in the return mud from the drill.

As used herein, “scavenging capacity” refers to the amount of sulfidesthat can be neutralized by the reaction between sulfides and scavengerto produce tied-up sulfides. Scavenging capacity is a measure of massper unit volume of sulfides.

As used herein, “lime” refers to calcium hydroxide.

As used herein, “scavenge” refers to the ability of the scavenger toreact with sulfides in the drilling mud.

Embodiments of the invention can include a method to neutralize hydrogensulfide in a contaminated drilling mud at the surface during drillingoperations. During the drilling of a well, a drilling mud can beinjected into the wellbore to aid the drill bit in drilling theborehole. The drilling mud can become contaminated by hydrogen sulfidereleased from the formation.

Referring to FIG. 1, a method to neutralize hydrogen sulfide in awellbore during drilling operations is provided. The figure andassociated method is described with respect to both start-up mode andsteady state operations. Unless otherwise indicated, the operating modeis steady state operations.

A pH buffer can be mixed with a drilling mud in rig tank 100. Rig tank100 can be any vessel suitable for preparing a drilling mud, andalternately for holding a drilling mud as the source for drillingoperations. In at least one embodiment, rig tank 100 is an open tank. Inat least one embodiment, rig tank 100 is a conventional rig tank and caninclude instrumentation and other mechanical devices, such as shakers.

Rig pump 115 can be used to pump and circulate drilling mud from rigtank 100 into drill 135 or wellbore 145.

During start-up mode, the pH buffer is mixed with the drilling mud toincrease the pH of the drilling mud to the target pH. During steadystate operations, the pH buffer is mixed with the drilling mud tomaintain the pH at the target pH. The target pH can be the same for bothstart-up mode and steady state operations. The target pH can be above10, alternately above 10.5, and alternately between 10.5 and 11.Hydrogen sulfide can dissolve in drilling mud. The solubility ofhydrogen sulfide in the drilling mud is inversely proportional to thepH. Advantageously, at a pH above 10, hydrogen sulfide dissolves indrilling mud and remains in solution. The hydrogen sulfide can producesoluble sulfide ions in the drilling mud. Without being bound to aparticular theory, it is understood that molecular hydrogen sulfide(H₂S) can alternate between the gas phase and liquid phase depending onthe pH of the carrying fluid. At high pH, such as pH above 10, thehydrogen sulfide can dissolve in the liquid phase and react withhydroxyl ions when dissolved, according to the following equation:

H₂S+OH⁻→HS⁻+H₂O.

The resulting bisulfide ion (HS−) can remain in solution due to itsnegative charge.

By maintaining the hydrogen sulfide in solution, corrosion problems areminimized because there is no free hydrogen sulfide gas. In at least oneembodiment of the present invention, hydrogen sulfide can stay dissolvedin the drilling mud by maintaining a pH at or above 10.5 throughout theentire system.

The steps of increasing the pH of the drilling mud with the pH bufferand allowing the hydrogen sulfide to dissolve in the drilling mud arenot alone sufficient to scavenge the hydrogen sulfide. In the absence ofa scavenger, a reduction in pH below 10 can result in hydrogen sulfidegas being released from solution in the drilling mud.

The dissolved hydrogen sulfide can reduce the pH of the drilling mud. Inat least one embodiment, an indication of the presence of hydrogensulfide in return flow line 150 can be determined based on the amount ofpH buffer mixed into rig tank 100.

The pH buffer can be selected from the group consisting of caustic soda,lime, monoethanolamine, and combinations thereof. Advantageously, andunlike caustic soda, lime can precipitate carbonates, which can form inthe presence of carbon dioxide. The amount of pH buffer added in rigtank 100 is determined by the need to adjust or maintain the pH of thedrilling mud.

The pH of the drilling mud can be measured by tank pH gauge 105. Tank pHgauge 105 can be any instrument capable of measuring the pH of thedrilling mud in rig tank 100. In at least one embodiment tank pH gaugeis any pH metering device. Tank pH gauge 105 provides a means to monitorthe pH of the drilling mud over time. In at least one embodiment of thepresent invention, the pH of the drilling mud is measured at tank pHgauge 105 every 30 minutes using a pen type pH meter. Adjustments to thequantity of pH buffer mixed into rig tank 100 can be made based on thepH measured by tank pH gauge 105.

During start-up mode, after the pH of the drilling mud is increased tothe target pH, the scavenger is added to rig tank 100 to produce ascavenger-containing drilling mud. In at least one embodiment, the pH ofthe drilling mud is maintained at the target pH so that the scavengercan react with the soluble sulfide ions. In at least one embodiment, thedrilling mud at the target pH can be circulated through the drill 135and wellbore 145 while the scavenger is added to rig tank 100.

The scavenger can any chemical compound that irreversibly reacts withhydrogen sulfide. In at least one embodiment, the scavenger can be aliquid-based triazine. In at least one embodiment, the liquid-basedtriazine is (hexahydro-1,3,5-triazine-1,3,5-triyl)triethanol marketedunder the trade name Mil-Gard® XPR by Baker Hughes®. In at least oneembodiment, the liquid-based triazine is a hydroxylalkyl triazinemarketed under the tradename Safe-Scav HS by M-I Swaco.

The scavenger reacts with the soluble sulfide ions in the drilling mudin an irreversible reaction to produce a tied-up sulfide. In at leastone embodiment, the tied-up sulfide remains in solution even when the pHof the drilling mud is reduced below 10. The tied-up sulfide is solublein the drilling mud regardless of pH. In at least one embodiment, thetied-up sulfide cannot be separated from the drilling mud. In at leastone embodiment, where the scavenger is(hexahydro-1,3,5-triazine-1,3,5-triyl)triethanol, the tied-up sulfide istriathine, produced according to the following reaction:

added to rig tank

capacity needed.

in

and the

pressure” refers to when the pressure exerted by the drilling fluid onthe formation is greater than the formation pressure, the difference isreferred to as the overbalance pressure. In at least one embodiment, thescavenging capacity of the amount of scavenger is in excess of thescavenging capacity needed to convert all of the hydrogen sulfidetied-up sulfide. In at least one embodiment of the present invention,the amount of scavenger is greater than 4 lb/bbl, alternately greaterthan 5 lb/bbl, alternately less than 6 lb/bbl, and alternately between 4lb/bbl and 6 lb/bbl. An amount of scavenger added to rig tank 100between 4 lb/bbl and 6 lb/bbl provides a scavenging capacity between 800mg/l and 1200 mg/l hydrogen sulfide in the scavenger-containing drillingmud. In at least one embodiment, the amount of scavenger is greater thanthe measured sulfides concentration.

The amount of scavenger can be tested at rig tank 100.

During steady state operations, the scavenging capacity in thescavenger-containing drilling mud can be determined with a Garrett GasTrain (GGT) instrument. A sample of the scavenger-containing drillingmud can be obtained from supply flow line 120. The sample is mixed witha known amount of sulfide to produce a known sulfide concentration, suchthat the known sulfide concentration is in excess of the suspectedscavenging capacity so that the known amount of sulfide reacts with aportion of the amount of scavenger in the sample. Any remaining sulfidesfrom the known amount of sulfide are measured by the GGT to determinethe measured sulfides concentration. The GGT instrument measures thesoluble sulfides in the sample to determine the measured sulfidesconcentration. The scavenging capacity of the scavenger-containingdrilling mud is determined based on the following equation:

scavenging capacity=[known sulfide concentration]−[measured sulfidesconcentration]

As an example, a sample of a scavenger-containing drilling mud is mixedwith a known amount of sulfides so that the known sulfide concentrationis 1500 mg/L. A GGT is then run to determine that the measured sulfidesconcentration is 700 mg/L. The scavenging capacity is then 800 mg/L(1500 mg/L−700 mg/L=800 mg/L). To measure the scavenging capacity of thereturn mud, it is assumed that some of the soluble sulfides have beenscavenged. A sample of the return mud is obtained. A known amount ofsulfides are mixed with the sample of return mud to achieve a knownsulfide concentration of 1500 mg/L in the sample of return mud. Then aGGT is run to determine that the measured sulfides concentration is 1200mg/L. The scavenging capacity of the sample of return mud was 300 mg/L(1500 mg/L−1200 mg/L=300 mg/L).

During steady state operations, the amount of scavenger to be added torig tank 100 is determined based on the scavenging capacity of thereturn mud in return flow line 150. The scavenging capacity can bedetermined based on a sample obtained from return flow line 150. In atleast one embodiment, the scavenging capacity is determined using aGarrett Gas Train (GGT) instrument as described. If the GGT measures adetectable amount of soluble sulfides as a measured sulfidesconcentration, that indicates the depletion of the scavenger in thereturn mud. If depletion of the scavenger is indicated, then an amountof scavenger can be added to rig tank 100 determined by the scavengingcapacity and the measured sulfides. In at least one embodiment, if thescavenging capacity in the return mud in return flow line 150 measuresat a value lower than the scavenging capacity in thescavenger-containing drilling mud then an additional amount of scavengeris added in rig tank 100. In at least one embodiment, an amount ofscavenger is added to rig tank 100 based on the measurements of thesample of return mud obtained from return flow line 150.

In at least one embodiment, the scavenging capacity is measured twiceper day. In at least one embodiment, the scavenging capacity is measuredas need to confirm scavenging capacity.

During start-up mode, after the target pH has been achieved and thedesired scavenging capacity is achieved, the scavenger-containingdrilling mud is allowed to flow from rig tank 100 through pump inlet 110to rig pump 115. Drill 135 operates to form wellbore 145 in formation140. The scavenger-containing drilling mud pumped through drill 135performs as a drilling mud to maintain pressure in wellbore 145 and tosweep out drilling cuttings as drill 135 operates. In addition, theamount of scavenger in the scavenger-containing drilling mud reacts withhydrogen sulfide that mixes with the scavenger-containing drilling mud.In at least one embodiment, hydrogen sulfide is released form theformation.

The reaction between the scavenger and the hydrogen sulfide can occur atany point in the system provided the pH is maintained between 10.5 and11.0. The reaction can occur at the bottom of wellbore 145, in annulus142, in return flow line 150 and in rig tank 100. After reaction occurs,the scavenger-containing drilling mud also contains tied-up sulfides.

After exiting drill 135, the scavenger-containing drilling mud travelsup annulus 142 and enters return flow line 150 as the return mud. Thereturn mud flows into rig tank 100, where more pH buffer and scavengercan be added as needed or as determined by measurements of samplesobtained from return flow line 150.

After the return mud flows at a steady rate, steady state operations isentered.

A method to neutralize hydrogen sulfide in a contaminated mud at asurface of a drilling site is provided. The contaminated mud from thewellbore is fed to a treatment loop where pH buffer is added to producea high pH mud at the target pH. Once the target pH is reached, scavengeris added to the high pH mud to produce a treated mud. The amount ofscavenger added is based on the measured sulfides concentration in thehigh pH mud. The treated mud is fed to at least one agitator tank whereagitators and a circulating pump create turbulence in the agitatortanks. The turbulence enhances the reaction between the scavenger andthe dissolved sulfides to produce tied-up sulfide. The reaction isenhanced because the turbulence creates better mixing, which increasesthe interaction between the sulfides and the scavenger. A scavengersample point in the agitator tanks can be used to obtain a sample tomeasure a measured sulfides concentration and when below an acceptablelimit, the treated mud exits the agitator tanks as a clean mud and flowsto the rig tank to be used in the rig with the drill.

The method to neutralize hydrogen sulfide provides a controlledenvironment to neutralize hydrogen sulfide.

Referring to FIG. 2, a system to neutralize hydrogen sulfide in acontaminated mud at the surface of the drilling site during drillingoperations is shown. A method to neutralize hydrogen sulfide in acontaminated mud at a surface during drilling operations is describedwith reference to FIG. 2.

A drilling mud is supplied to drill 135 from rig tank 100. Hydrogensulfide present in wellbore 145 is carried from formation 140 y thedrilling mud to produce a contaminated mud.

The contaminated mud with hydrogen sulfide is fed to treatment loop 200through treatment line 250. Treatment loop 200 includes buffer injectionpoint 202, sample point 204, and scavenger injection point 206.Treatment loop 200 is designed to allow for a retention time intreatment loop 200 of between 1 minute and 3 minutes, and alternatelyless than 5 minutes. As used herein, “designed” refers to pipe diameter,length, and configuration such that the desired parameter. In at leastone embodiment, treatment loop 200 is secured by support skids (notshown).

Buffer injection point 202 can be any type of port capable of allowingthe pH buffer to enter treatment loop 200. Buffer injection point 202can include a reservoir (not shown) containing the pH buffer forinjection, alternately, buffer injection point 202 can be connected to apH buffer source by piping. The pH buffer is injected into thecontaminated mud at buffer injection point 202 to produce a high pH mud.

Sample point 204 can be any type of port capable of allowing a sample tobe drawn from treatment loop 200. In at least one embodiment, samplepoint 204 can include an instrument for measurement to allow real-timemeasurement of the high pH mud. In at least one embodiment, sample point204 includes a pH meter. At sample point 204, the pH of the high pH mudis measured. If the pH is below the target pH, additional pH buffer isinjected at buffer injection point 202. If additional pH buffer isinjected, a second measure of pH is taken at sample point 204 todetermine the pH of the high pH mud. The cycle of injecting additionalpH buffer at buffer injection point 202 and measuring the pH at samplepoint 204 can continue until the target pH is reached. As describedabove with reference to FIG. 1, when the high pH mud reaches the targetpH, hydrogen sulfide can dissolve into solution in the high pH mud. Themeasured sulfides concentration can be measured from the sample obtainedfrom sample point 204

Scavenger injection point 206 can be any type of port capable ofallowing the scavenger to enter treatment loop 200. Scavenger injectionpoint 206 can include a reservoir (not shown) containing the scavengerfor injection, alternately, scavenger injection point 206 can beconnected to a scavenger source by piping. At scavenger injection point206, an amount of scavenger is injected into the high pH mud to producea treated mud.

The treated mud exits treatment loop 200 and flows to agitator tank 220through treated mud line 210. Agitator tank 220 can include one or moretanks. Each agitator tank 220 includes at least one agitator 225. Thetreated mud flows into agitator tank 220, where agitators 225 createturbulence in agitator tank 220. The turbulence increases the mixing ofthe scavenger in the treated mud, which enhances the reaction betweenthe scavenger and the hydrogen sulfide to produce tied-up sulfides.

In at least one embodiment, circulating pump 230 can be connected toagitator tank 220 to further mix the scavenger in agitator tanks 225.The treated mud is fed to circulating pump 230 through pump suction 232.The treated mud is returned to agitator tank 220 through a mixing hopper(not shown) and through pump outlet 234. As shown in FIG. 2, when two ormore agitator tanks are used, valves (denoted by V in a circle) can beused to direct the flow around circulating pump 230 and agitator tanks220. The flow rate through circulating pump 230 is based on the wellboresize and rate of cleaning as determined by best practices.

Each agitator tank 220 can include a buffer sample point 222 and ascavenger sample point 224. Buffer sample point 222 can be any type ofinstrument capable of measuring the pH of a fluid. If the pH, asmeasured at buffer sample point 222, is below the target pH, thenadditional pH buffer can be added at buffer injection point 202.Scavenger sample point 224 can be any type of port that allows a sampleto be obtained from agitator tank 220. In at least one embodiment, thesample obtained from scavenger sample point 224 can be analyzed in a GGTinstrument to measure the measured sulfides concentration and determinethe scavenging capacity of the fluid in agitator tank 220.

Advantageously, the use of agitator tanks, agitators, and a circulationpump provides sufficient reaction time for the scavenger to neutralizethe hydrogen sulfide.

When the measured sulfides concentration is less than 10 mg/L, or lessthan the allowed amount based on local regulations then the treated mudexits as clean mud through clean mud line 240 and flows to rig tank 100,where it can be used with rig 130 and drill 135.

The system to neutralize hydrogen sulfide at a drill site can alsoinclude flow meters, pressure sensors, and other instrumentation usefulin monitoring a process. Advantageously, the system to neutralizehydrogen sulfide can provide continuous scavenging of hydrogen sulfidewithout having to stop drilling due to risk of hydrogen sulfide release.Advantageously, the system provides full control of the scavengingcapacity of a drilling mud due to continuous measurement of the measuresulfides concentration in the drilling mud.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions, and alterations canbe made hereupon without departing from the principle and scope of theinvention. Accordingly, the scope of the present invention should bedetermined by the following claims and their appropriate legalequivalents.

The singular forms “a,” “an,” and “the” include plural referents, unlessthe context clearly dictates otherwise.

Optional or optionally means that the subsequently described event orcircumstances can or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such a range isexpressed, it is to be understood that another embodiment is from theone particular value and/or to the other particular value, along withall combinations within said range.

As used herein and in the appended claims, the words “comprise,” “has,”and “include” and all grammatical variations thereof are each intendedto have an open, non-limiting meaning that does not exclude additionalelements or steps.

As used herein, terms such as “first” and “second” are arbitrarilyassigned and are merely intended to differentiate between two or morecomponents of an apparatus. It is to be understood that the words“first” and “second” serve no other purpose and are not part of the nameor description of the component, nor do they necessarily define arelative location or position of the component. Furthermore, it is to beunderstood that that the mere use of the term “first” and “second” doesnot require that there be any “third” component, although thatpossibility is contemplated under the scope of the present invention.

What is claimed is:
 1. A method to neutralize hydrogen sulfide in acontaminated mud at a surface during drilling operations, the methodcomprising the steps of: introducing the contaminated mud to a treatmentloop, where the contaminated mud exits a wellbore prior to beingintroduced to the treatment loop, wherein the treatment loop comprises:a pH buffer injection point, the pH buffer injection point configured toinject a pH buffer into the contaminated mud to produce a high pH mud ata target pH, a sample point, the sample point configured to measure thepH of the high pH mud, and a scavenger injection point, the scavengerinjection point configured to inject an amount of scavenger; injectingpH buffer into the contaminated mud at the buffer injection point toproduce the high pH mud; obtaining a sample of the high pH mud from thesample point; measuring a pH of the sample of the high pH mud; comparingthe pH of the sample of the high pH mud to a target pH; injecting theamount of scavenger into the high pH mud to produce a treated mud whenthe pH of the sample of high pH mud is below the target pH; introducingthe treated mud into an agitator tank, the agitator tank configured tocirculate the treated mud for a treatment time to produce a clean mud,wherein the amount of scavenger can be further mixed with the treatedmud in the agitator tank; and introducing the clean mud to a rig tank,the rig tank configured to supply the clean mud to the wellbore.
 2. Themethod of claim 1, wherein the amount of scavenger is in excess of 4lb/bbl.
 3. The method of claim 1, wherein the scavenging capacitybetween 800 mg/l and 1200 mg/l.
 4. The method of claim 1, wherein theagitator tank comprises: an agitator, the agitator configured to createturbulence in the agitator tank; and a circulation pump, configured towithdraw a portion of the treated mud and return the withdrawn portionto the agitator tank.
 5. The method of claim 1, wherein the sample pointcomprises a pH monitor.
 6. The method of claim 1, wherein the scavengeris triazine.
 7. The method of claim 1, further comprising the steps of:collecting a sample of the treated mud from the agitator tank; mixing aknown amount of sulfides in the sample to produce a known sulfideconcentration in the sample, wherein the known amount of sulfide reactswith a portion of the scavenger in the sample to leave a remainingamount of sulfides, measuring the remaining amount of sulfides in thesample to produce a measured sulfides concentration, wherein thescavenging capacity is the known sulfide concentration minus themeasured sulfides concentration; and adding an additional amount ofscavenger at the scavenger injection point if the scavenging capacity isbelow a depleted scavenger level.